Washington: US may have lost one of its most advanced drones over Iran, raising fears among officials here that Tehran may gain partial access to closely guarded American technology.

Tehran claimed yesterday that it had shot down an advanced RQ-170 drone in eastern Iran and these unmanned surveillance planes are among the most sensitive in the American fleet, New York Times reported.

The paper said that RQ-170 drones had been deployed only recently and the stealth version of the UAV was flown for hours on repeated occasions over Osama bin Laden's hideout in Abbottabad in Pakistan earlier this year, without being detected by Pakistani air defences.But the shooting down of this drone could compromise US technology to Washington's arch foe Tehran. NYT said for months there have been unconfirmed reports that the same drone was being used regularly over Iran, presumably to hunt for hidden nuclear or missile sites.

In a statement on Sunday, US-led International Security Assistance Force (ISAF) in Afghanistan said that the drone "to which the Iranians are referring maybe an US unarmed reconnaissance aircraft that had been flying a mission over western Afghanistan late last week."

The ISAF statement said that operators of the remotely controlled drone aircraft lost control of it. But it did not say, what kind of drone was lost or what might have caused the loss.

The shooting down of the drone comes as Los Angeles Times, quoting former US intelligence officials, said that last month's explosion at a military base near Tehran was part of a covert effort by the US, Israel and other states to disable Iran's nuclear and missile programme.

The newspaper said the goal of the covert effort is to derail Iran's quest for nuclear weapons capability and to stave off Israeli or US air strikes to eliminate or lessen the threat.

NYT quoting US and Israeli intelligence officials said that the huge explosion that destroyed a major missile testing site near Tehran three-weeks ago was a major setback for Iran's most advanced long range missile program.

The report said that surveillance photos of the base showed it was a testing centre for advanced solid fuel missiles and the base was completely levelled in the blast.

Solid fuel missiles, US experts said, can be launched almost instantly, making them useful to Iran as a potential deterrent against preemptive strikes.

Missiles powered by solid fuels rather than liquids have no need for trucks to fill them with volatile fluids and can be fired on short notice, making it hard for other nations to destroy before they are launched.